The present invention pertains to a plasma processing apparatus useful in plasma-assisted processes such as etching, deposition or sputtering processes. More particularly, the invention relates to an Inductive Coupled Plasma source.
Inductive Coupled Plasma sources are conventionally used for etching substrates or depositing material thereon. They are generally composed of a plasma chamber having a lateral dielectric wall surrounded by an external solenoid inductor. The plasma chamber contains a process gas. The external inductor is fed by a Radio Frequency generator and produces electromagnetic energy which is provided to the interior of the plasma chamber through the dielectric wall. A plasma is thus generated inside the chamber, by ionization of the process gas molecules.
The composition of the process gas is selected as a function of the treatment to be applied to the substrate. Thus, for example, if the plasma apparatus is used for material deposition on a substrate, the process gas will contain material elements to be deposited on this substrate. If the plasma apparatus is used as an etching apparatus, the process gas will contain reactive species that will react with the substrate and remove material therefrom.
An interesting characteristic of the plasma-assisted processes, in particular with respect to the Chemical Vapour Deposition technique, is that, by virtue of the use of the plasma, a lower substrate temperature is required in order to activate the reaction or deposition. Plasma-assisted processes may therefore be advantageously applied notably when the substrate to be treated cannot withstand high temperatures.
However, Inductive Coupled plasma sources remain expensive, in particular when they are shaped so as to have curved portions. The material from which their external wall is made, i.e. a dielectric material, is indeed not appropriate for the manufacture of complex pieces, such as curved or cylindrical pieces. Furthermore, drilling a hole in the dielectric material, for example in order to provide an access to the interior of the chamber for the process gas, is a difficult task.
WO-A-96/41897 describes a plasma treatment apparatus comprising a parallelepiped plasma chamber surrounded by electrical conductors. The side walls of the plasma chamber are composed of parallel metallic bars extending longitudinally over the length of the chamber. Gaps or slots are defined between the metallic bars in order to allow the magnetic field produced by the electrical conductors to penetrate into the interior of the plasma chamber. The plasma chamber is sealed by dielectric plates, each covering the external side of a corresponding chamber side wall.
The use of a dielectric plate for each side wall of the chamber may pose a problem when it is desired to have a chamber with a more complex shape, such as a cylindrical shape. Curved dielectric plates would indeed have to be constructed, which would significantly increase the manufacturing costs. Furthermore, the plasma chamber according to WO-A-96/41897 is not appropriate for providing a lateral access for the process gas, since such lateral access would require drilling a hole through a dielectric plate.
The present invention aims at remedying the above-mentioned drawbacks.
To this end, there is provided a plasma processing apparatus comprising a plasma chamber comprising an electrically conductive wall, wherein said electrically conductive wall has at least one aperture for interrupting a current path in said wall when electromagnetic energy is provided into the chamber from the outside of the chamber, external electromagnetic means for supplying electromagnetic energy into said plasma chamber through said electrically conductive wall in order to create a plasma inside the chamber, and sealing means for sealing said at least one aperture, characterised in that said sealing means comprises at least one electrically conductive enclosure element which is electrically insulated from said electrically conductive wall.
Thus, according to the present invention, one or more electrically conductive enclosure elements are used to seal the aperture(s) defined on the conductive wall of the plasma chamber. The plasma chamber is therefore essentially made from an electrically conductive material, such as metal, which renders the construction of the chamber more flexible. In particular, the chamber may have a complex shape, without the manufacturing costs significantly increasing, since conductive materials may be machined and deformed rather easily.
Furthermore, as the conductive enclosure elements are electrically insulated from the conductive wall, a current path through the conductive wall is interrupted at the level of the aperture(s), thus preventing a current loop to be formed all around the conductive wall when electromagnetic energy is supplied to the interior of the chamber. By virtue of the provision of the aperture(s) and of the fact that the conductive enclosure element(s) is insulated from the conductive wall, the electromagnetic energy produced by the external electromagnetic means is not or little screened out by the conductive wall. An important portion of the electromagnetic energy produced by the external electromagnetic means is therefore effectively supplied to the inside of the chamber.
Preferably, said at least one aperture extends in a first direction which is substantially parallel to the magnetic field induced by the external electromagnetic means, so as to interrupt a current path through the electrically conductive wall in a direction orthogonal to said first direction.
In practice, the electrically conductive wall extends in a direction parallel to a longitudinal axis of the plasma chamber, and said at least one aperture extends in a direction which is substantially parallel to the longitudinal axis of the plasma chamber, so as to interrupt a current path through said electrically conductive wall in a direction orthogonal to said longitudinal axis.
The plasma chamber may further comprise end plates at opposite ends of the electrically conductive wall, which end plates extend perpendicularly to the conductive wall.
The plasma chamber may be cylindrical, with the electrically conductive wall forming the cylindrical face of the plasma chamber, or parallelepiped, with the electrically conductive wall being composed of plane adjacent faces of the chamber. The plasma chamber may also be of any other suitable shape, and in particular of a shape that is adapted to the shape of the substrate to be plasma treated.
The external electromagnetic means may comprise an inductor surrounding the electrically conductive wall and connected to an AC power supply.
According to the present invention, at least one insulating means or electrically non-conductive gasket is provided for electrically insulating each conductive enclosure element from the conductive wall.
Preferably, each non-conductive gasket is disposed on the conductive wall in such a manner as to be in indirect line of sight from the interior of the chamber, i.e. from the plasma generated inside the chamber. This feature is particularly advantageous when the plasma processing apparatus according to the invention is used for depositing or sputtering electrically conductive material, such as metal. Indeed, in such a case, there would be a risk that a continuous layer of such conductive material be coated on the non-conductive gasket when the plasma deposition or sputtering process is being carried out. This could result in the creation of a current loop all around the conductive wall of the chamber and, accordingly, in the electromagnetic energy supplied by the external electromagnetic means being screened out by this continuous layer in the manner as explained above.
By avoiding any direct line of sight between the interior of the chamber and the insulating gasket, the risk of energy screening out due to conductive material deposition on the chamber wall is prevented or at least greatly reduced. This result is attained without the present invention requiring any Faraday shield within the chamber, nor any additional complex labyrinth structure, as conventionally used in the prior art. Furthermore, as the enclosure element(s) is/are made of a conductive material, such as metal, the deposition of conductive material thereon is not detrimental. The present invention makes it thus possible to use wide apertures or slots on the conductive wall in order to simplify the construction of the apparatus and to reduce its cost.
Typically, said at least one aperture consists of a plurality of apertures, said at least one electrically conductive enclosure element consists of a plurality of electrically conductive enclosure elements, and each one of the plurality of conductive enclosure elements is mounted on the electrically conductive wall in registration with a corresponding one of the plurality of apertures.
According to an embodiment of the present invention, said at least one gasket consists of a plurality of gaskets, each one of the plurality of gaskets surrounding, on an external surface of the electrically conductive wall, a corresponding one of the plurality of apertures, and each one of the plurality of electrically conductive enclosure elements is applied on a corresponding one of the plurality of gaskets.
According to another embodiment of the present invention, each one of the plurality of apertures comprises a narrow portion emerging into the plasma chamber and a wider portion communicating with the narrow portion and forming a recess on an external surface of the electrically conductive wall. Each one of a plurality of gaskets surrounds, in the recess of a corresponding aperture, the narrow portion of the corresponding aperture, and each one of the plurality of conductive elements is applied on a corresponding gasket in the recess of a corresponding aperture.
Advantageously, the at least one gasket and the at least one conductive enclosure element are removably mounted on the conductive wall. Preferably, a gap between the at least one conductive enclosure element and the conductive wall is determined according to the Paschen""s law, such that no plasma is generated between the conductive enclosure element(s) and the conductive wall. Typically, the gap may be less than one millimeter.
The plasma chamber may further comprise one or several sputtering targets. These sputtering targets may be disposed on an inner surface of the conductive wall. An insulating plate may be interposed between the sputtering targets and the inner surface of the conductive wall. The sputtering source may be connected to a power source by a lead crossing the conductive wall and electrically insulated from the conductive wall.
According to another advantageous feature of the present invention, the plasma chamber comprises at least one gas inlet formed on the electrically conductive wall. This gas inlet serves to supply the inside of the chamber with a process gas. Preferably, the at least one gas inlet consists of a plurality of gas inlets provided at regular intervals on the electrically conductive wall. The process gas can thus be introduced into the chamber with a uniform distribution.
In addition to the above-mentioned gas inlets, the plasma chamber may further comprise one or more gas inlets formed on a cover plate disposed at the top or the bottom of the chamber. The gas inlets on the conductive wall may then serve to supply the interior of the chamber with a reactive gas mixture, while the gas inlets provided on the cover plates may be used for introducing inert gases whose role notably consists in facilitating the mixing of the various elements contained in the gas mixture. The gas inlets provided on the conductive wall, which forms a lateral wall of the chamber, enable the introduction of the reactive gas mixture into the chamber in an area close to the substrate to be treated. In this way, the present invention avoids supplying the reactive gas mixture to areas which are remote from the substrate and which therefore need not receive the gas mixture.
In practice, the at least one electrically conductive enclosure element preferably consists of one or several plates, and both the conductive wall and the plates are made of metal. Furthermore, the at least one aperture may take the form of one or several slots.